As part of the complete design cycle, CAD representations must ultimately be converted into physical forms including prototype parts and tooling to manufacture the parts. The ability to quickly fabricate physical shapes, to minimize the time and cost required to reiterate designs, is one key to manufacturing competitiveness. A sprayed metal tooling system provides a method to rapidly build custom tooling directly from prototype patterns quickly produced, for instance, with stereolithography apparatus (SLA). The sprayed metal tooling process uses thermal spraying, for instance, electric arc spraying whereby metal wire is fed to a torch, melted in an electric arc, gas atomized, and sprayed onto a substrate surface. On contact, the sprayed material solidifies and forms a surface coating. Spray coatings can be built up by depositing multiple bonded layers which, when separated from the substrate, form a free-standing shell with the shape of the substrate surface. By mounting the shell in a frame, and backing it with appropriate materials, a die can be fabricated. For example, the cavities of prototype injection models can be fabricated by direct deposition of zinc metal onto plastic SLA models of the desired part and backing the frame shell with epoxy resins. Sprayed metal tooling is described more fully in U.S. Pat. No. 3,533,271 to Burbank et al.
Steel, however, cannot be effectively deposited by the sprayed metal tooling process. In practice, during the spraying of the molten steel, residual stress is created in the steel shell as it solidifies. This residual stress causes the steel to peel away from the substrate as new layers are applied. Not only steel but many other alloys with high melting temperatures exhibit this problem.
Zinc and zinc alloys, in contrast, can be sprayed to significant thicknesses with nominal shrinkage to form an accurate shell. Previous processes for making sprayed steel tooling required that release agents, such as PVA, first be applied to the patterns. The release agent helps to hold the sprayed metal in place. The patterns could be machined or made with solid-freeform-fabrication techniques. However, PVA is not effective in holding sprayed steel in place because of the extremely high residual stress associated with sprayed steel. Thus, sprayed steel tends to peel off the pattern after a thin layer has been built-up. In addition, PVA tends to burn-off when high melting point metals, such as steel, are sprayed onto it.
In the present invention, a low melting point metal is first sprayed onto the pattern. The low melting point metal can withstand the heat of the rapidly solidifying sprayed metal and, in fact, it helps conduct the heat away. More importantly, the low melting point metal clamps the steel down, by local anchoring, until a backing material can be poured to mechanically stabilize the sprayed shell. The present invention provides a method for accurately forming steel shells on a pattern, such as those produced with a stereolithography apparatus, by resisting shrinkage due to high residual stresses within the shell during a spray tooling process.